After a year of working with Swift, I've settled down to my own metric: Swift code (given my coding styles in both languages) is 30% the length of Objective-C code that does the same thing.

Another point that isn't widely noted -- Swift is easier to teach than Objective-C. A couple of months ago, I taught a course in iOS app development to newbies, and I used both Swift and Objective-C. By the second day, I settled on the strategy of teaching the Swift version first. It makes it easy for people who don't know Cocoa/Cocoa Touch or one of the Apple languages to get the hang of things quickly. After that, going back to show how to reimplement something in Objective-C is a lot faster than it used to be.